In high-frequency applications, coaxial connectors are typically used to connect either devices or transmission lines to other transmission lines. The coaxial cable used for the transmission lines in these applications have a characteristic impedance which is defined as the impedance that would be presented at the input terminals of a transmission line that is theoretically infinitely long. An open circuit anywhere along the transmission line represents an end of the transmission line which will reflect the transmitted signal back towards the input terminals or the source.
A matching circuit is typically employed to solve The reflection problem. The matching circuit typically consists of a resistance equal to that of the characteristic impedance of the cable which is placed at the cable end between the signal and the shield. For example, a typical characteristic impedance for a coaxial line is 50 Ohms; therefore, a 50 Ohm resistor can be used for the matching circuit and is connected between the signal and the shield at the end of the coaxial transmission line.
Since an unmated coaxial connector represents the end of the transmission line in a circuit, a termination plug containing the matching circuit is typically connected to the unmated coaxial connector. The termination plug serves to connect a resistance equal to the characteristic impedance of the cable between the signal contact and the shield of the open unmated coaxial connector.
This presents a problem in complex circuits having many coaxial connectors which may be either in the mated or unmated condition during operation. All of the unmated coaxial connectors would require a termination plug to be connected thereto in order to avoid any reflection of the high frequency signals back towards a source. If one unmated connector is overlooked or if the termination plug is lost, undesired back reflection will result.
Known methods of addressing this problem include placing a normally closed switch into the unmated coaxial connector which will close a circuit between the signal contact and the shield having a resistor equal in resistance to the characteristic impedance of the cable. Weber teaches such connectors in U.S. Pat. Nos. 5,108,300 and 5,320,546. These patents show a pair of switch contacts, one of which has an impedance element connected between one of its ends and the outer shell or shield of the connector. The other switch contact is connected to the signal contact on a printed circuit board. The switch is normally closed such that in the unmated condition the signal contact is connected to the outer shell of the connector through the impedance element. Upon mating, the signal contact is separated from the other switch contact and is then connected to the signal contact of the mating connector.
There are several problems with this design; the first being that in high-frequency applications it is desirable to maintain a coaxial relationship between the signal and the shield contacts when the connector is in the unmated condition. It will be noted here that the switch contacts are not in a coaxial orientation, instead they are simply adjacent to each other. This has an adverse effect on the electrical performance of the connector when operated at high frequency. Wherever the coaxial orientation is not maintained, there will be a change in impedance in that area. The impedance will be lower in the coaxial areas than in the switching area. As a signal passes through the connector, there will be reflections at every transition between the higher and lower impedance areas results in increased signal losses through the connector. Also since the switching action relies on a lateral motion, there is a tendency for the biased switch contact to apply a normal force to the center contact of the mating connector. This normal force may be sufficient to bring the center contact of the mating connector in contact with the switch contact which is connected to the shell thus causing a short circuit.
U.S. Pat. No. 5,237,293 discloses a self-terminating coaxial cable connector having a switchable impedance equal to the characteristic impedance of a coaxial connector. The switch of this connector, like the Weber connectors, operates by a lateral force exerted on the switch contact in order to open the switch. The switch contact here is actuated by a housing edge surface of the mating connector. This edge surface may be damaged when the mating connector is in the unmated condition which would adversely affect actuation of the switch upon mating. The switch contact also exerts lateral forces on the center contact much like the Weber patents which could possibly affect proper centering location of the center contact. The problem with all of these connectors is that they exert a lateral force on the center contact of the connectors. Additionally, they typically have a ground path length that is longer than necessary between the signal contact and the connection to the shield through the impedance element. This adversely affects the electrical performance of the coaxial connector.